Last month, BBC1 announced that inoffensive con caper Hustle will be returning for an eighth series. Eighth? Yes, eighth. Let mild shock be unconfined. The jazzy crime romp is one of a clump of programmes that are repeatedly recommissioned by channel nabobs, without anyone really knowing why. Unchallenged and unremarked upon, they're just there, like clouds or carpet. Watching them is a wholly passive experience. It is possible to view an entire series of New Tricks, for example, and still be unable to tell the difference between Dennis Waterman and a barrel of singing luncheon meat. While other shows perish after a series or two, Hustle, New Tricks and their virus-like brethren return year after year, stubbornly resistant to the vagaries of fashion. So what do they have that other series haven't? Should – or, indeed, can – they be stopped? Here's a rough guide to the Shows That Cannot Be Killed.

New Tricks (BBC1, 2003-)

Amanda Redman. Photo: BBC/Wall To Wall

Harmless old buffers tackle uncomplicated cold cases while swatting irritably at the vagaries of modern life with their orthopaedic truncheons.

Hours of Crimes solved over corn plasters and Thermoses of milky tea. Encounters with fashion/youth/electrical goods made after 1982 leading to much head-scratching and attendant in-my-day harrumphage. Bumbling in anoraks. Fumbling in slacks. Pantomime baddies. Wilting scripts. Dennis Waterman's dentures. All enveloped by a fog of nostalgia.

Why it's still going The warm, buttery plots and familiar, approachable cast remind older viewers of the days when they could buy a pint for tuppence ha'penny and still have change for a misjudged June Whitfield cameo.

Chances of cancellation Slim. Consistently high ratings suggest any attempt to axe the bugger off at the knees will be met with eight million mottled fists, each wielding a rolled-up, mildew-ridden copy of Reader's Digest.

Silent Witness (BBC1, 1996-)

Emilia Fox shows and tells. Photo: BBC/Justin Canning

Attractively upholstered pathologists deliver damning forensic diagnoses over a succession of spooned-out prosthetic cadavers to the consternation of table-thumping top brass.

Hours of Intensive corridor acting accessorised with splash-proof clipboards and subplot-ready scowls. Unfocused close-ups of Dr Harry Cunningham's jawline inserted into every third scene for maximum "phwoarensic" impact. Dialogue of the "there are lesions to the orbitofrontal cortex" genus.

Why it's still going Gravitas. Or rather "gravitas". There's nothing like an unusual camera angle or outbreak of distressed piano to convince the viewers that what they're watching is important, as opposed to merely, say, an actor in disposable overshoes shaking his head at a photograph of a compound fracture.

Chances of cancellation Low. Robust ratings prevail, while the departure of Amanda "middle distance" Burton in 2004 has seen wilful abstruseness replaced by a slicker, Spooks-ier, less overtly guff approach.

Hours of 13.5 tog murders pondered amid snoozing spires and the distant peal of pre-recorded evensong. DS Hathaway mulling solemnly over long-lens snaps of shifty businessmen. DI Lewis squashing up his face like a confused grapefruit and asking the barman for another crab-paste sandwich.

Why it's still going One word: Morse. The presence of Whately, Oxford, Morse creator Colin Dexter (as series consultant) and Morse composer Barrington Pheloung suggests a more appropriate title for the spin-off would be Inspector Morse II: The Dullening.

Chances of cancellation Minuscule. Its parentage has bestowed Lewis with Grade I-listed Heritage Drama status. Tamper with such a monument and you might as well be drawing a penis on the Cenotaph.

Midsomer Murders (ITV1, 1997-)

The mean streets of Midsomer. Photo: Mark Bourdillon

DCI Barnaby trudges between social engagements accompanied by the steady thwack-thwack of blunt instrument against Home Counties skull.

Hours of Cufflinks. Corpses retrieved from woodland glade, fireside rug and village fete. Ne'er-do-wells dodging capture amid clouds of pollen and airborne theremins. Men in pastel linen looking over their shoulders before entering a cricket club. Scones.

Why it's still going Dogged adherence to British TV crime tradition. That is: violent murders in rural locales solved by busybodies in wide-fitting shoes. And, of course, no black people to spoil the views. Lovely.

Chances of cancellation 2/1. While the full impact of John Nettles's departure has yet to be ascertained, ill-chosen remarks by (now ex-) producer Brian True-May re: the programme's status as a "bastion of Englishness" have hinted at something more sinister lurking in the Midsomer undergrowth. Will its extraordinary popularity overseas provide a stay of execution? Possibly.

Antiques Roadshow (BBC1, 1979-)

Antiques Roadshow's John Sandon. Photo: BBC

Britain endures appalling toilet facilities in a string of regional sports centres in order to have its heirlooms fondled by toffs with fingers like battered sausages.

Hours of Queues. Pensioners peering over Fiona Bruce's shoulder at some sort of clock. Lawns. Eric Knowles's bow tie oscillating with excitement as he informs Mrs McStupefied of Kirkcaldy that he hasn't seen a finer pith helmet since the annexation of West Griqualand.

Why it's still going The fusion of history, knowledge and manners speaks of gentler times and nobler ways, of humility, tweed, bicycle bells and feigned interest in the provenance of one's scrimshaw.

Chances of cancellation Sod all. Extract this from the schedules and Sundays will collapse like a bombed occasional table. Such is its popularity that it will probably last for ever, presented by a hologram of Fiona Bruce's head and wisecracking robot sidekick E-VALU8.

Taggart (STV/ITV1, 1983-)

The Taggart-less cast of Taggart. Photo: SMG

PC Plods with complexions like desiccated farm slurry apprehend sundry local "characters" against a backdrop of porridge-grey Glasgow drizzle.

Hours of Rain. Accents. Implacable DCI Burke (chief grump since the death of Mark "Jim Taggart" McManus in 1994) verbally debagging joyless DI Reid over a misplaced lever arch file, before storming off to interrogate someone called Rab.

Chances of cancellation Critical. ITV announced in May that it would not be commissioning any further series. Yet producers STV remain unbowed, with talk of "exploring a range of options with other broadcasters" and "commitment to the brand". This commitment to artistic integrity at last offers hope to those whose heart soars at the prospect of watching DI Ross tuck into another steak bake in a Honda Civic.